This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous inkjet printers wherein a liquid ink stream breaks into droplets, some of which are selectively deflected.
The printing technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d inkjet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no printing is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as a catcher, an interceptor, or a gutter). When printing is desired, the ink droplets are directed to strike a print media.
Typically, continuous inkjet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
U.S. Pat. No. 1,941,001, titled xe2x80x9cRecorder,xe2x80x9d issued Dec. 26, 1933 to C. W. Hansell, and U.S. Pat. No. 3,373,437, titled xe2x80x9cFluid Droplet Recorder With A Plurality Of Jets,xe2x80x9d issued Mar. 12, 1968 to R. G. Sweet et al. each disclose an array of continuous inkjet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous inkjet printing.
U.S. Pat. No. 3,416,153, titled xe2x80x9cInk Jet Recorder,xe2x80x9d issued Dec. 10, 1968 to C. H. Hertz et al. discloses a method of achieving variable optical density of printed spots in continuous inkjet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small aperture.
U.S. Pat. No. 3,878,519, titled xe2x80x9cMethod And Apparatus For Synchronizing Droplet Formation In A Liquid Stream,xe2x80x9d issued Apr. 15, 1975 to James H. Eaton discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, titled xe2x80x9cMethod And Apparatus For Controlling The Electric Charge On Droplets And Ink-Jet Recorder Incorporating The Same,xe2x80x9d issued Aug. 24, 1982 to Carl H. Hertz discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.
U.S. Pat. No. 4,638,382, titled xe2x80x9cPrinthead For An Ink Jet Printer,xe2x80x9d issued Jan. 20, 1987 to Donald J. Drake et al. discloses a continuous inkjet printhead that utilizes constant thermal pulses to agitate ink streams admitted through a plurality of nozzles in order to break up the ink streams into droplets at a fixed distance from the nozzles. At this point, the droplets are individually charged by a charging electrode and then deflected using deflection plates positioned in the droplet path.
As conventional continuous inkjet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes to operate effectively. This results in continuous inkjet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, titled xe2x80x9cMethod And Apparatus For Aerodynamic Switching,xe2x80x9d issued Jan. 9, 1973 to John A. Robertson discloses a method and apparatus for stimulating a stream of ink causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitude stimulations resulting in longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow effects the trajectories of the filaments before they break up into droplets more than it effects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
While this method does not rely on electrostatic means to effect the trajectory of droplets, it does rely on the precise control of the break up points of the filaments and the placement of the air flow intermediate to these break up points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small, further adding to the difficulty of control.
U.S. Pat. No. 4,190,844, titled xe2x80x9cInk-let Printer With Pneumatic Deflector,xe2x80x9d issued Feb. 26, 1980 to Terrence F. E. Taylor discloses a continuous inkjet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. Similar arrangements are also disclosed in Soviet Union Patent No. 581478, titled xe2x80x9cInked Recording Of Pneumatic Signals On Paper Tape Using Pulsed Pressure Droplet Stream And Deflecting Nozzle For Signal,xe2x80x9d issued Nov. 29, 1977 and in European Patent No. 494385 issued Jul. 15, 1992 to Dietrich et al. A printhead supplies a stream of ink that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9cON/OFFxe2x80x9d type having a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount that a nozzle is open, depending on a varying electrical signal received at the central control unit. The second pneumatic deflector oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at time, and are built up by repeated traverses of the printhead.
While this method does not rely on electrostatic means to effect the trajectory of droplets, it does rely on the precise control and timing of the first (xe2x80x9cON/OFFxe2x80x9d) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture especially for high-nozzle count printheads since independent pneumatic actuators are required for each inkjet. In addition, electromechanical actuators which would be typically used to modulate the air flow have slow response times. Consequently, the printing of individual drops, according to image data, would be very slow, relative to other commercialized inkjet printheads in the current marketplace. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic, due to the precise timing requirements; hence, increasing the difficulty of controlling printed and non-printed ink droplets and resulting in poor ink droplet trajectory control.
Additionally, using two pneumatic deflectors complicates construction of the printhead and requires more components. The additional components and complicated structure require large spatial volumes between the printhead and the media, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and effects the print image quality. Again, there is a need to minimize the distance that the droplet must travel before striking the print media in order to insure high quality images.
U.S. Pat. No. 6,079,821, titled, xe2x80x9cContinuous Ink Jet Printer With Asymmetric Heating Drop Deflection,xe2x80x9d issued Jun. 27, 2000 to James M. Chwalek et al. discloses a continuous inkjet printer that uses actuation of asymmetric heaters to create individual ink droplets from a stream of ink and to deflect those ink droplets. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While the inkjet printer disclosed in U.S. Pat. No. 6,079,821 (Chawlek et al.) works extremely well for its intended purpose, it is best adapted for use with inks that have a large viscosity change associated with temperature. Each of the above-described inkjet printing systems has advantages and disadvantages. However, printheads which require low-power and low-voltages to operate are advantageous in the marketplace, especially in page-width arrays. The use of heaters to break up the ink streams into droplets has significant advantages over a piezo-transducer (as described in U.S. Pat. No. 4,350,986 titled xe2x80x9cInk Jet Printer,xe2x80x9d issued Sep. 21, 1982 to Takahiro Yamanda) in that the heaters can be made in a much more compact structure than the piezo-transducer type, which permits a larger density of nozzles per inch, and significantly lower manufacturing costs for the heater design. In addition, the use of heaters permits the volumes of either large or small drops to be easily adjusted and controlled, whereas droplets formed by a piezo-type vibrator are not easily adjustable and are highly dependent on the fluid properties of the ink, such as surface tension and viscosity.
U.S. Pat. No. 5,455,614 titled xe2x80x9cPrinting Method And Print Head Having Angled Ink Jet,xe2x80x9d issued Oct. 3, 1995 to Paul M. Rhodes discloses a system in which a continuous inkjet printhead assembly is angled, relative to the print substrate, such that the printing droplets follow a more perpendicular path toward the substrate. In this method, both the plane of the ink nozzle and also the plane of the deflection means are tipped to achieve the desired printing angle. This approach can be applied when the path length from the nozzle to the print media is relatively long, however, if the path length is short (for example, 3-4 mm), there would be insufficient room to angle a nozzle plate and a gas-flow deflector away from their previously used orientation, which is parallel to the print media.
International Application published under the Patent Cooperation Treaty (PCT), WO 81/03149, published Nov. 12, 1981, discloses a continuous inkjet apparatus in which electrostatic droplet deflection is used to discriminate between printing and non-printing droplets. Additionally, a second electrode structure is used to alter the path of printing drops so they strike the print media at a perpendicular angle. Good droplet placement is then achieved for printing on non-smooth or wrinkled surfaces. While this method solves the problem of non-perpendicular droplet paths, it requires that the ink droplets be charged which leads to drop-drop repulsion artifacts. In addition, the method requires high voltages and expensive control circuitry, and necessitates that the inks be within a certain conductivity range.
Referring to FIG. 1, a prior art continuous inkjet printer system 5 is shown. The prior art continuous inkjet printer system 5 includes an image source 10 such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to half-toned bitmap image data by an image processing unit 12, which also stores the image data in memory 13. A heater control circuit 14 reads data from the image memory 13 and applies electrical pulses to a heater 32 that is part of a printhead 16. These pulses are applied at an appropriate time, so that drops formed from a continuous inkjet stream will print spots on a recording medium 18 in the appropriate position designated by the data in the image memory. The printhead 16, shown in FIG. 1, is commonly referred to as a page width printhead.
Recording medium 18 is moved relative to printhead 16 by a recording medium transport system 20 which is electronically controlled by a recording medium transport control system 22, and which in turn is controlled by a micro-controller 24. The recording medium transport system 20 shown in FIG. 1 is a schematic only, and many different mechanical configurations are possible. For example, a transfer roller could be used as recording medium transport system 20 to facilitate transfer of the ink drops to recording medium 18. Such transfer roller technology is well known in the art. In the case of page width printheads 16, it is most convenient to move recording medium 18 past a stationary printhead 16.
Ink is contained in an ink reservoir 28 under pressure. In the nonprinting state, continuous inkjet drop streams are unable to reach recording medium 18 due to an ink gutter 34 that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit 36. The ink recycling unit 36 reconditions the ink and feeds it back to the ink reservoir 28. Such ink recycling units 36 are well known in the art. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzle bores (shown in FIG. 2) and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to ink reservoir 28 under the control of ink pressure regulator 26. System 5 can incorporate additional ink reservoirs 28 in order to accommodate color printing. When operated in this fashion, ink collected by the ink gutter 34 is typically collected and disposed.
The ink is distributed to the back surface of printhead 16 by an ink channel 30. The ink preferably flows through slots and/or holes etched through a silicon substrate of printhead 16 to its front surface where a plurality of nozzles and heaters are situated. With printhead 16 fabricated from silicon, it is possible to integrate heater control circuits 14 with the printhead. Printhead 16 can be formed using known semiconductor fabrication techniques (CMOS circuit fabrication techniques, micro-electro mechanical structure MEMS fabrication techniques, etc.). Printhead 16 can also be formed from semiconductor materials other than silicon.
Referring to FIG. 2, printhead 16 is shown in more detail. Printhead 16 includes a drop forming mechanism 38. Drop forming mechanism 38 can include a plurality of heaters 40 positioned on printhead 16 around a plurality of nozzle bores 42 formed in printhead 16. Although each heater 40 may be disposed radially away from an edge of a corresponding nozzle bore 42, heaters 40 are preferably disposed close to corresponding nozzle bores 42 in a concentric manner. Typically, heaters 40 are formed in a substantially circular or ring shape. However, heaters 40 can be formed in other shapes. Typically, each heater 40 comprises a resistive heating element 44 electrically connected to a contact pad 46 via a conductor 48. A passivation layer is normally placed over the resistive heating elements 44 and conductors 48 to provide electrical insulation relative to the ink. Contact pads 46 and conductors 48 form a portion of the heater control circuits 14 which are connected to micro-controller 24. Alternatively, other types of heaters can be used with similar results.
Heaters 40 are selectively actuated to form drops, for example, as described in U.S. patent application Ser. No. 09/751,232. The volume of the formed droplets is a function of the rate of ink flow through the nozzle and the rate of heater activation, but is independent of the amount of energy dissipated in the heaters. FIG. 3 is a schematic example of the electrical activation waveform provided by micro-controller 24 to heaters 40. In general, rapid pulsing of heaters 40 forms small ink droplets, while slower pulsing creates larger drops. In the example presented here, small ink droplets are to be used for marking the image receiver, while larger, non-printing droplets are captured for ink recycling.
In this example, multiple drops per nozzle, per image pixel are created. Periods P0, P1, P2, etc. are the times associated with the printing of associated image pixels, the subscripts indicating the number of printing drops to be created during the pixel time. The schematic illustration shows the drops that are created as a result of the application of the various waveforms. A maximum of two small printing drops is shown for simplicity of illustration, however, the concept can be readily extended to permit a larger maximum count of printing drops.
In the drop formation for each image pixel, a non-printing large drop 95, 105, or 110 is always created, in addition to a selectable number of small, printing drops. The waveform of activation of heater 40 for every image pixel begins with electrical pulse time 65. The further (optional) activation of heater 40, after delay time 83, with an electrical pulse 70 is conducted in accordance with image data wherein at least one printing drop 100 is required as shown for interval P1. For cases where the image data requires that still another printing drop be created as in interval P2, heater 40 is again activated after delay 84, with a pulse 75. Heater activation electrical pulse times 65, 70, and 75 are substantially similar, as are all delay times 83 and 84. Delay times 80, 85, and 90 are the remaining times after pulsing is over in a pixel time interval P and the start of the next image pixel. All small, printing drops 100 are the same volume. However, the volume of the larger, non-printing drops 95, 105 and 110 varies depending on the number of small drops 100 created in the preceding pixel time interval P as the creation of small drops takes mass away from the large drop during the pixel time interval P. The delay time 90 is preferably chosen to be significantly larger than the delay times 83, 84 so that the volume ratio of large, non-printing drops 110 to small, printing drops 100 is a factor of about 4 or greater.
It can be seen that there is a need for improved drop placement as controlled by conventional inkjet printheads that employ a gas flow deflector for separating droplets into printing and non-printing paths. More specifically, there is a need to retain the features of low-power and low-voltage printhead operation in a continuous inkjet printhead while providing an improved printing droplet path relative to the print media.
The aforementioned need is met according to the present invention by providing a method for printing ink droplets that strike print media substantially perpendicularly, including the steps of: emitting a first drop having a first volume and a second drop having a second volume as a stream of ink from a plurality of nozzle bores formed in a printhead; moving either the first drop or the second drop into a substantially perpendicular strike position relative to the print media; separating either the first drop or the second drop along different droplet paths; capturing either the first drop or the second drop with an ink gutter; and striking the print media with either the first drop or the second drop substantially perpendicular to the print media.
Another aspect of the present invention provides an apparatus for printing an image wherein printable droplet paths are perpendicular to an image receiver, that includes: a printhead including: one or more nozzles from which streams of ink droplets of adjustable volumes are emitted; a first droplet deflector adapted to produce a force on the streams of ink droplets, the force being applied to the streams of ink droplets at an angle to cause the streams of ink droplets having a first range of volumes to move along a first set of paths, and streams of ink droplets having a second range of volumes to move along a second set of paths; a controller adapted to adjust the streams of ink droplets emitted by the one or more nozzles according to image data to be printed; an ink catcher positioned to allow the streams of ink droplets moving along the first set of paths to move unobstructed past the ink catcher, while intercepting the streams of ink droplets moving along the second sets of paths, and; a second droplet deflector which alters the flight path of the streams of ink droplets having a first range of volumes so that the flight path becomes perpendicular to the image receiver.